1. Field of the Invention
The present invention relates to a method for producing polylactic acid pellets substantially containing no low-molecular components, such as lactide, and an apparatus used therefor.
2. Discussion of the Related Art
Polylactic acid is a biologically very safe polymer, and its degradated product, namely lactic acid, is absorbed in vivo. Having the above properties, polylactic acid is useful for medical purposes, including surgical sutures, sustained-release capsules in drug delivery systems, and reinforcing materials for bone fractures. Moreover, it is noted as a biodegradable plastic, because it is degraded under natural environmental conditions. It is also widely used for monoaxially and biaxially stretched films, fibers, extrusion products, and various other purposes.
Known methods for producing a polylactic acid are as follows: In one method, lactic acid is directly subject to dehydration condensation to give a desired product. In another method, a cyclic lactide of lactic acid, is first synthesized from lactic acids and then purified by such methods as crystallization, followed by ring-opening polymerization. Various procedures for synthesizing, purifying and polymerizing lactide are disclosed in U.S. Pat. No. 4,057,537, EP-A-261,572, Polymer Bulletin, 14, pp.491-495 (1985), Makromol. Chem., 187, 1611-1628 (1986), and other chemistry literatures. Also, JP-B-56-14688 discloses a method for producing a polylactic acid comprising polymerizing a bimolecular cyclic diester, as an intermediate, using tin octylate or lauryl alcohol as a catalyst. Also, polylactic acid can be produced directly from lactic acid by the methods as described in JP-7-33861, JP-59-96123, and The Proceedings of The Discussion on Macromolecules, vol.44, pp.3198-3199. The polylactic acid thus obtained is pelletized into various forms, such as spheres, cubes, columns, and disrupted pieces, in a size of from a rice grain to a bean, to facilitate its handling in the forming process.
However, polylactic acid having a high molecular weight of from 100,000 to 500,000 has a high melting point of from 175.degree. to 200.degree. C. Conventionally, when the final polymer product of the above polylactic acid is taken out from the reactor in a molten state and heated to a temperature equal to or higher than its melting point, the polylactic acid undergoes decomposition and coloration. Moreover, a large amount of lactide is generated in the polymer by heating to the above-mentioned temperature range, presumably owing to the fact that a polymer-lactide equilibrium is shifted toward the lactide side at the above temperature.
These lactide and decomposition products thereof are liable to sublime during injection molding or spinning of the polylactic acid pellets used as a starting material and undesirably adhere to dice or nozzles, and thereby the operation is hampered. Furthermore, the lactide and the decomposition products lower the glass transition temperature and the melt viscosity of the polymer, thereby resulting in drastic deterioration of moldability and thermal stability.
In view of the above problems, various methods for removing low-molecular components of the polylactic acids have been proposed, including Japanese Patent Laid-Open No. 3-14829. This publication discloses that residual low-molecular components or low-molecular volatile components can be removed by keeping a polyester at a temperature between its melting point and 250.degree. C. and under a reduced pressure of 5 mmHg or less. It further discloses that when the temperature is lower than its melting point, substantially no unchanged monomer components become volatile. In this method the low-molecular components are removed in a molten state, the polymer-lactide equilibrium is therefore shifted toward the lactide side, thereby resulting in the regeneration of the lactide in the product polymers. Therefore, this method has notable limitation in the removal of the low-molecular components.
Also, Japanese Patent Laid-Open No. 5-255488 discloses a method for increasing a molecular weight of a low-molecular weight lactic acid polymer, comprising the steps of heating the starting low-molecular weight lactic acid polymer having an individual particle size of from 5 .mu.m to 5 mm at a temperature higher than its glass transition temperature and lower than its melting point, and carrying out condensation polymerization by dehydration reaction. However, in this method, the operation required about 240 hours according to the examples thereof, thereby making it impossible to carry out industrially. Moreover, this method was mainly intended to be applied to materials for medical use. Therefore, it has a feature of increasing the molecular weight without using a catalyst, and is never directed to reducing the low-molecular components.